1. Field of the Invention
The present invention relates to a band-pass filter, and more particularly to a band-pass filter including LC parallel resonators and trap resonators.
2. Description of the Related Art
A band-pass filter including LC parallel resonators and trap resonators, such as that described in Japanese Unexamined Patent Application Publication No. 2002-94349, has been known.
In the band-pass filter, as illustrated in FIG. 6, an LC parallel resonator including an inductor L1 and a capacitor C1, which are connected in parallel, and an LC parallel resonator including an inductor L2 and a capacitor C2, which are connected in parallel, are connected between an input terminal P1 and an output terminal P2. In addition, a trap resonator including an inductor L3 and a capacitor Ct1, which are connected in parallel, is connected in series between the input terminal P1 and the LC parallel resonator including the inductor L1 and the capacitor C1, which are connected in parallel. Further, a trap resonator including an inductor L4 and a capacitor Ct2, which are connected in parallel, is connected in series between the output terminal P2 and the LC parallel resonator including the inductor L2 and the capacitor C2, which are connected in parallel. These trap resonators are provided to secure a sufficient attenuation amount necessary for desired frequencies outside the passband.
When the trap resonators are connected in series between the input and output terminals and the LC parallel resonators, as illustrated in FIG. 6, impedance mismatch occurs between the input and output terminals and the LC parallel resonators. Therefore, it is necessary to overcome this mismatch when the band-pass filter described in Japanese Unexamined Patent Application Publication No. 2002-94349 is connected to an external circuit. To this end, adjusting elements, such as capacitors and/or inductors, for overcoming the impedance mismatch are necessary to be additionally provided between the LC parallel resonators and the trap resonators.